US20080044126A1 - Integrated Optical Waveguide Structure with Low Coupling Losses to an External Optical Field - Google Patents

Integrated Optical Waveguide Structure with Low Coupling Losses to an External Optical Field Download PDF

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US20080044126A1
US20080044126A1 US10/566,948 US56694803A US2008044126A1 US 20080044126 A1 US20080044126 A1 US 20080044126A1 US 56694803 A US56694803 A US 56694803A US 2008044126 A1 US2008044126 A1 US 2008044126A1
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waveguide
waveguide core
width
optical
refractive index
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Raffaella Costa
Giuseppe Cusmai
Andrea Melloni
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Pirelli and C SpA
Google LLC
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/122—Basic optical elements, e.g. light-guiding paths
    • G02B6/1228—Tapered waveguides, e.g. integrated spot-size transformers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/30—Optical coupling means for use between fibre and thin-film device
    • G02B6/305—Optical coupling means for use between fibre and thin-film device and having an integrated mode-size expanding section, e.g. tapered waveguide
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B2006/12083—Constructional arrangements
    • G02B2006/12097—Ridge, rib or the like

Definitions

  • the present invention generally relates to planar integrated optical waveguides, and, more particularly, to integrated optical waveguides having medium to high refractive index contrast values.
  • Low refractive index contrast integrated optical waveguides i.e., waveguides characterized by a refractive index contrast of less than approximately 1%) have traditionally been used in integrated optical devices because, having relatively wide cross-sections, the dimensions of the optical modes supported by these waveguides are comparable to those of standard optical fibers; consequently, high coupling efficiencies are ensured when the integrated waveguides are coupled to optical fibers (fiber-to-waveguide coupling efficiency).
  • fiber-to-waveguide coupling efficiency fiber-to-waveguide coupling efficiency
  • the optical power transferred from one optical guiding structure to the other strongly depends on how well the optical modes supported by each of the two optical guiding structures overlap.
  • the overlap integral between the modes supported by the two guiding structures is usually taken as a measure of the coupling efficiency.
  • High integration scales have been achieved using medium to high refractive index contrast integrated waveguides, which are characterized by refractive index contrast values higher than 1%, up to approximately 40%, depending on the specific application.
  • These integrated waveguides allow fabricating very compact devices, because waveguide patterns with small bending radii, down to few microns, can be formed without incurring in high losses.
  • high refractive index contrast integrated waveguides are made of semiconductor materials, such as, for example, InGaAsP/InP and AlGaAs/GaAs.
  • Semiconductor waveguides feature refractive index differences larger than 1 ⁇ 10 ⁇ 2 (by comparison, in glass optical fibers the refractive index difference is usually less than 5 ⁇ 10 ⁇ 3 ).
  • the use of this kind of waveguides poses problems in terms of losses when the waveguides are coupled to optical fibers.
  • the waveguides in order to guarantee single-mode operating conditions in high refractive index contrast waveguides, the waveguides must have rather small cross sections, which implies small optical field dimensions.
  • the dimensions ratio between the mode in the waveguide and that in a fiber coupled thereto can be very low, and the overlap integral between the modes supported by the two guiding structures drops to very low values.
  • the fiber-to-waveguide coupling losses need to be reduced to acceptable values.
  • spot-size conversion structures have been proposed for adapting (“converting”) the spot size in the waveguide to that in the fiber.
  • Most of these structures implement combined multilayer laterally and vertically tapered waveguide structures, designed to convert the waveguide field shape into the fiber mode. Examples of these structures are provided in U.S. Pat. No. 6,240,233, describing an integrated optical beam spread transformer for a InGaAsP/InP waveguide, in the technical manuscript “Design and Fabrication of Monolithic Optical Spot Size Transformer (MOST's) for Highly Efficient Fiber-Chip. Coupling” by G. Wenger et al, published in the IEEE Journal Of Lightwave Technology, Vol. 12, No.
  • SiON silicon oxynitride
  • pages 86 to 87 describes the design of a mode-size adapter for a SiON on SiO 2 waveguide having an index contrast equal to 0.24 (in percentage approximately 16%), consisting of a laterally tapered SiON waveguide having a step-wise decrease in thickness towards the taper point, which may have up to 0.5 ⁇ m residual width.
  • planar spot-size converter structures are required.
  • Planar spot size converters using periodic, quasi-periodic or non-periodic segmented waveguides have been proposed, as reported for example in the technical paper “A Very Short Planar Silica Spot-Size Converter Using a Nonperiodic Segmented Waveguide” by M. M. Spuehler et al, published in the IEEE Journal Of Lightwave Technology, Vol. 16, No. 9, September 1998, pages 1680 to 1685, which describes a planar spot-size converter structure designed and implemented in a SiO 2 /SiON system of materials.
  • a process for manufacturing an integrated optical waveguide structure comprising:
  • forming a waveguide core on the lower cladding layer wherein said forming the waveguide core comprises:
  • a waveguide core rib protruding from a surface of the waveguide core layer opposite to a surface thereof facing the lower cladding layer, said waveguide core rib, having a substantially uniform height, the waveguide core rib having a layout defining a path for the guided optical field.
  • Said forming the waveguide core rib further comprises:
  • At least one coupling waveguide portion designed for coupling an external optical field to a circuit waveguide portion in which the waveguide core rib has a first width.
  • Said forming the at least one coupling waveguide portion comprises in turn:
  • terminal waveguide core rib portion having a second width lower than the first width and terminating
  • Simpler planar spot-size converters consist in a waveguide that is only laterally tapered, having a lateral width that varies along a transition section thereof, possibly according to an optimized profile, towards an optimized value at an interface facet with an optical fiber, so as to maximize the overlap integral in the fiber-to-waveguide coupling.
  • simple laterally tapered waveguide mode converters are based on the fact that, when the waveguide width is decreased below a given value, the width of the mode supported by the waveguide increases; thus, narrowing the waveguide towards the interface facet until a waveguide mode dimension comparable to the fiber mode dimension is attained allows achieving a high fiber-to-waveguide coupling efficiency, while preserving single-mode operation.
  • a mode-shape converter having upper and lower optical rib waveguides including a substrate, a lower cladding coated over the substrate, a lower rib waveguide, a core, an upper rib waveguide and an upper cladding.
  • the lower rib waveguide defines a stepped pattern existing partially only in a coupling region and in a conversion region.
  • tapered waveguides have been employed in combination with other structures, as for example described in the European patent application No. EP 1 245 971 A2, describing the provision of lateral rib confinement waveguides extending laterally to a tapered rib waveguide.
  • Tapered waveguides have also been exploited in applications different from the realization of fiber-to-integrated waveguide coupling structures; for example, the International patent application No. WO 02/42808 A2 describes the use of a tapered waveguide for forming an optical waveguide multimode-to-single mode transformer, for interfacing a laser, having a multi-mode output, to a single-mode optical fiber.
  • the mode transformer has a high refractive index core layer, e.g. made of SiON, surrounded by a lower refractive index cladding.
  • the core layer includes a wide input waveguide section to accept a multimode, including a fundamental mode, light input.
  • the input waveguide section is coupled to a narrow output waveguide section by a tapered region having taper length enabling adiabatic transfer of the fundamental mode of the multimode light from the wide input waveguide section to the output waveguide section while suppressing (stripping) other modes.
  • the narrow output waveguide section supports a single mode light output comprising the fundamental mode.
  • the input waveguide section and the tapered region comprises a ridge waveguide, having a ridge on the core layer, with a width of the ridge decreasing in the tapered region.
  • the lateral margins of the silicon nitride core are etched through to form a real index guided structure.
  • the integrated waveguide characteristics have to fulfill several requirements: for example, the waveguide geometric dimensions typically have to guarantee a monomodal operation, the refractive index contrast has to be chosen so as to minimize radiation losses in bends and allow high integration density, the material birefringence must be compensated by means of form birefringence or in other ways.
  • suitable solutions to the problem of fiber-to-waveguide coupling losses need to be devised, so as to keep the losses at an acceptably low level. All these requirements are to be fulfilled with an eye at the fabrication process.
  • tapered waveguide mode adapters known in the art are either too complicated to be manufactured, or the design thereof is difficult to be optimized, or, in the case of the simple laterally tapered buried waveguides, only the fiber-to-chip coupling efficiency is optimized (i.e., attention is mainly paid to the width of the tip of the buried waveguide and to the shape of the transition region), without taking in the due consideration the other circuital requirements that need to be satisfied.
  • ridge or rib integrated waveguides are to be preferred over other integrated waveguide structures, such as buried waveguides, because they offer to the designer of integrated optical devices a higher flexibility.
  • rib waveguides it is easier to design integrated waveguides that are optimized both from the viewpoint of the coupling efficiency with an external optical field, for example for coupling the integrated optical device with optical fibers, and from the viewpoint of the other waveguide circuital requirements.
  • the Applicant has realized that, when dealing with medium to high refractive index contrast structures, i.e., structures with refractive index contrast values ranging from approximately 1% to approximately 40% and, preferably, from approximately 1% to approximately 20%, rib waveguides are preferable to other integrated waveguide structures for the reason that the presence of the slab offers a further degree of freedom to the designer, and a favorable tradeoff between the different requirements is more easily reached.
  • the slab height can be exploited to enable the material birefringence compensation in favor of polarization-insensitive operation.
  • a thick slab allows high coupling coefficients and wider gaps in directional couplers in favor of higher tolerance to the technological process; on the other hand, an excessive slab height causes high radiation losses in small radii bends of the waveguides; the slab thickness also influences the coupling efficiency with optical fibers and the single mode operation.
  • the different physical and geometrical parameters can be used to meet a set of different requirements: integration density, bending radiation losses, single mode condition, mode dimensions and so on.
  • an integrated optical waveguide structure as set forth in claim 1 .
  • the integrated waveguide structure comprises a waveguide core for guiding an optical field, the waveguide core being formed on a lower cladding layer;
  • the waveguide core comprises a waveguide core layer substantially coextensive to the lower cladding layer and having a substantially uniform thickness, and a waveguide core rib, of substantially uniform height, protruding from a surface of the waveguide core layer opposite to a surface thereof facing the lower cladding layer, a layout of the waveguide core rib defining a path for the guided optical field.
  • substantially coextensive means that the waveguide core layer has a surface extension sufficiently wide so that the optical field in the waveguide core layer is substantially equal to zero proximate to the borders of the waveguide core layer.
  • the waveguide core layer has a size of at least two times the maximum width at 1/e of the local optical field.
  • the surface extension of the waveguide core layer is such as not to substantially affect the lateral confinement of the light, the light lateral confinement being instead provided only by the waveguide core rib.
  • the integrated optical waveguide structure comprises a circuit waveguide portion in which the waveguide core layer has a first width, adapted to guiding the optical field through an optical circuit, and at least one coupling waveguide portion adapted to coupling the circuit waveguide portion to an external optical field.
  • the coupling portion comprises a terminal waveguide core rib portion having a second width lower than the first width and terminating in a facet, and a transition waveguide core rib portion optically joining to each other the waveguide core rib of the circuit waveguide portion and the terminal waveguide core rib portion.
  • the transition waveguide core rib portion is laterally-tapered so that a width thereof decreases from the first width to the second width.
  • a ratio between the second width and the first width, and a ratio between the height of the waveguide core layer and an overall height of the waveguide core are chosen in such a way as to keep coupling losses arising when the external optical field is coupled to the integrated waveguide below a prescribed level.
  • At least one among a value of the first width, a value of the overall height of the waveguide core and a value of the height of the waveguide core layer is chosen in such a way as to comply with requirements on the circuit waveguide portion depending on the optical circuit; at least one among a value of the second width and a value of the height of the waveguide core layer is instead chosen in such a way as to achieve a prescribed efficiency in the coupling of the integrated waveguide to an external optical field having first field dimensions.
  • the circuit waveguide portion may be designed to support an optical field of second field dimensions equal to or lower than the first field dimensions; the coupling waveguide portion performs a field dimensions adaptation for adapting the second field dimensions to the first field dimensions.
  • the circuit waveguide portion is designed in such a way as to support a single-mode optical field.
  • this is not strictly necessary, because single-mode excitation in the circuit waveguide portion is ensured by the limited width of the terminal waveguide core rib portion.
  • the integrated circuit waveguide is designed in such a way that a ration of the first field dimensions to the second field dimensions falls in the range from approximately 1 to approximately 3.
  • the lower cladding layer has a first refractive index
  • the waveguide core has a second refractive index
  • an upper cladding covering the waveguide core has a third refractive index; in a preferred embodiment of the invention the first, second, and third refractive indexes are such that a refractive index contrast between the waveguide core and the lower and upper claddings falls in the range from approximately 1% to approximately 20%, more preferably in the range from approximately 5% to approximately 7%.
  • the waveguide core is made of silicon oxynitride (SiON); the lower cladding layer is made of silicon dioxide; the upper cladding may be made of silicon dioxide or gas, e.g. air.
  • SiON silicon oxynitride
  • the lower cladding layer is made of silicon dioxide
  • the upper cladding may be made of silicon dioxide or gas, e.g. air.
  • a length of the transition waveguide core rib portion is chosen in dependence of a ratio between the first width and the second width.
  • a length is chosen to be at least equal to a minimum length that, expressed in microns, is given by the formula (1 ⁇ W/W 0 )*500.
  • the terminal waveguide core rib portion preferably has a length chosen to be the shortest possible length taking account of technological tolerances in a process of separating a die in which the optical waveguide structure is integrated from other dies formed from a same wafer.
  • the length of the terminal waveguide core rib portion may be determined on the basis of said minimum length and of the length of the transition waveguide core rib portion.
  • the length of the terminal waveguide core rib portion is chosen to be approximately equal to a value that, expressed in microns, is given by the formula L tec exp( ⁇ (L/L min ) 2 ), where L tec denotes a length depending on said technological tolerances and L min is said minimum length.
  • the coupling method comprises providing at least one coupling waveguide portion, designed for coupling an external optical field to a circuit waveguide portion in which the waveguide core rib has a first width.
  • the coupling waveguide portion comprises a terminal waveguide core rib portion having a second width lower than the first width and terminating in a facet, and a transition waveguide core rib portion optically joining to each other the waveguide core rib in the circuit waveguide portion and the terminal waveguide core rib portion.
  • the transition waveguide core rib portion being laterally-tapered so that a respective width decreases from the first width to the second width.
  • a ratio between the second width and the first width, and a ratio between the height of the waveguide core layer and an overall height of the waveguide core are chosen in such a way as to keep coupling losses arising when the external optical field is coupled to the integrated waveguide below a prescribed level.
  • At least one among a value of the first width, a value of the overall height of the waveguide core and a value of the height of the waveguide core layer may be chosen in such a way as to comply with requirements on the circuit waveguide portion depending in a facet, and
  • transition waveguide core rib portion optically joining to each other the waveguide core rib in the circuit waveguide portion and the terminal waveguide core rib portion, said transition waveguide core rib portion being laterally-tapered so that a respective width decreases from the first width to the second width.
  • said forming the waveguide core comprises:
  • the terminal portion and the transition portion are formed simultaneously with said forming of the waveguide core rib.
  • FIG. 1 is a schematic illustration of a planar integrated optical waveguide according to an embodiment of the present invention
  • FIG. 2 is a diagram showing the variation of the coupling efficiency between two circular gaussian optical fields (in ordinate), one in an optical fiber and the other in an integrated optical waveguide, as a function of the ratio of the field diameters at 1/e (in abscissa, logarithmic scale);
  • FIG. 3 is a diagram showing the width and height at 1/e (in ordinate) of an optical mode supported by the waveguide of FIG. 1 , as a function of a width of the waveguide (in abscissa);
  • FIGS. 4A , 4 B and 4 C show contour plots of waveguide-to-fiber coupling losses simulated for the waveguide of FIG. 1 as a function of the ratio of a waveguide core layer height to the overall waveguide core height (t/h, in ordinate) and of the ratio of the width of a waveguide tip to a width of a waveguide circuit portion (W/W 0 , in abscissa) of a waveguide according to an embodiment of the present invention, for three different values of refractive index contrast and for a fixed ratio of fiber-to-waveguide mode dimensions;
  • FIG. 5A is a diagram similar to those of FIGS. 4A , 4 B and 4 C, showing contour plots of average coupling losses calculated from the coupling losses values depicted in the diagrams of FIGS. 4A , 4 B and 4 C;
  • FIG. 5B is a diagram similar to those of FIGS. 4A , 4 B and 4 C showing the standard deviations of the coupling losses from the average coupling losses reported in FIG. 5A .
  • FIGS. 6A to 6C show coupling losses contour plots diagrams similar to those of FIGS. 4A , 4 B and 4 C, simulated for the waveguide of FIG. 1 for two different values of the ratio of fiber-to-waveguide mode dimensions, and for a fixed value of refractive index contrast, equal to that corresponding to the diagram of FIG. 4B ;
  • FIGS. 7A and 7B are diagrams showing the measured coupling efficiencies (in ordinate, dB scale) as a function of the fiber-to-waveguide misalignment along the horizontal axis and, respectively, the vertical axis (in abscissa, ⁇ m); and
  • FIG. 8 schematically depicts an exemplary integrated optical device, in which a waveguide structure according to an embodiment of the present invention is exploited.
  • FIG. 1 a planar integrated optical waveguide structure according to an embodiment of the present invention is schematically shown. More precisely, only a small portion of a waveguide 101 is depicted in the drawing, namely a waveguide portion proximate to an edge or tip 103 of the waveguide 101 , intended to be coupled to, e.g., an optical fiber 105 (more generally, to an external optical field, either guided or not).
  • an optical fiber 105 more generally, to an external optical field, either guided or not.
  • the waveguide 101 is integrated in a chip 107 in which one or more optical components (not shown in FIG. 1 ) can also be integrated.
  • CVD Chemical Vapor Deposition
  • PECVD Plasma-Enhanced CVD
  • a waveguide core 113 is formed, having a refractive index n core .
  • the waveguide core 113 made for example of silicon oxynitride (SiON), having a refractive index n core that falls in the range from approximately 1.45 to approximately 2, is formed by depositing a SiON layer on the lower cladding layer 111 , e.g. by CVD and, particularly, PECVD.
  • the deposited SiON layer is patterned, so as to form a core base layer (in jargon, a slab) 113 a , of substantially uniform height t throughout the die, and, on the core base layer 113 a , a core ridge or rib 113 b , of height (h ⁇ t), where h denotes the overall height of the waveguide core 113 .
  • RIE Reactive Ion Etching
  • a birefringence compensating layer (not shown in the drawing) can be formed, interposed between the lower cladding layer 111 and the waveguide core 113 ;
  • the birefringence compensating layer may be made of silicon nitride (Si 3 N 4 ), formed by Low-Pressure CVD (LPCVD).
  • the upper cladding 115 of refractive index n uc covers the waveguide core 113 .
  • the upper cladding 115 can be a material layer, for example made of SiO 2 , similarly to the lower cladding layer 111 (in which case the upper cladding refractive index n uc and the lower cladding refractive index n 1c coincide).
  • the upper cladding 115 can be made of, e.g., air (refractive index n uc equal to 1), or other fluid or gas.
  • an optical field 121 propagates through the waveguide 101 being guided by and being substantially confined within the waveguide core 113 .
  • the waveguide core rib 113 b confines the optical field 121 upperly and laterally, and the layout pattern thereof determines the optical field path in a plane parallel to that of the core base layer 113 a.
  • the waveguide core rib 113 b has a substantially uniform height (h ⁇ t) throughout the die.
  • the waveguide core rib 113 b has instead a variable width in different regions of the chip 107 .
  • the waveguide core rib 113 b has a circuit waveguide core rib portion 117 a , of prevailing length, which is the portion of the waveguide intended to interact with the optical device or devices integrated in the chip 107 ; the circuit waveguide core rib portion 117 a has a first width (circuit waveguide width) W 0 .
  • a laterally-tapered, transition waveguide core rib portion 117 b joins the circuit waveguide portion 117 a to a tip waveguide core rib portion 117 c , of length L tip and having a second width (tip waveguide width) W lower than the circuit waveguide width W 0 .
  • the tip waveguide core rib portion 117 c terminates in a facet 119 (typically, but not limitatively, a facet coincident with the chip perimetral boundary; more generally, a interface facet between a region of the space in which the layer 113 is present, and an adjacent region of space in which the layer 113 is absent, for example in correspondence of a groove formed in an area of the chip), through which the waveguide 101 can be interfaced to an external optical field, e.g. carried by the optical fiber 105 , or can emit optical radiation.
  • a facet 119 typically, but not limitatively, a facet coincident with the chip perimetral boundary; more generally, a interface facet between a region of the space in which the layer 113 is present, and an adjacent region of space in which the layer 113 is absent, for example in correspondence of a groove formed in an area of the chip
  • the waveguide core rib can have a larger width; by way of example, in an embodiment of the present invention, the width in the circuit waveguide core rib portion 117 a can be the maximum width that still guarantees the single-mode operating condition.
  • the circuit waveguide portion has a strong guiding action, at least for the fundamental optical mode, while the tip waveguide, having a reduced core rib width, has a weak guiding action on the fundamental mode.
  • the profile and the length L of the laterally-tapered transition waveguide core rib portion 117 b are chosen to avoid abrupt transitions between the narrower tip waveguide core rib portion 117 c and the wider circuit waveguide core rib portion 117 a.
  • the length L and the profile of the laterally-tapered transition waveguide core rib portion 117 b may be determined according to any known design procedure, for example the one described in the already cited technical manuscript “Design of a Single-Mode Tapered Waveguide for Low-Loss Chip-to-Fiber Coupling” by O. Mitomi et al, published in the IEEE Journal of Quantum Electronics, Vol. 30, No. 8, August 1994, pages 1787 to 1793, the content of which is incorporated herein by reference.
  • planar integrated waveguide structure depicted in FIG. 1 offers to the integrated optical device designer a great flexibility in the task of designing an integrated waveguide that satisfies the requirements in terms of both circuit waveguide characteristics and coupling efficiency with, e.g., an optical fiber.
  • the height t of the slab 113 a and the width W 0 of the circuit waveguide core rib portion 117 a can be chosen in such a way as to satisfy circuit requirements for the waveguide, i.e., requirements deriving from the interaction of the waveguide with the optical devices integrated in the chip 107 , the designer is left free to determine at least one among the height t of the slab 113 a and the tip waveguide width W in such a way as to optimize the coupling efficiency between the waveguide and a selected optical fiber, having a given mean mode diameter.
  • a rib waveguide structure i.e., a waveguide structure in which the waveguide core comprises a core base layer, or slab, 113 a , of uniform thickness, and core rib 113 b , offers the possibility of designing and fabricating waveguides that are optimized in respect to the circuit requirements, and, by means of simple, laterally-tapered mode spot-size conversion structures, are also optimized in respect of the coupling efficiency with external fields.
  • an overlap integral between the modes supported by the two guiding structures is defined as follows:
  • ⁇ [ ⁇ e f ⁇ ( x , y ) ⁇ e wg ⁇ ( x , y ) ⁇ ⁇ x ⁇ ⁇ y ] 2 ⁇ e f 2 ⁇ ( x , y ) ⁇ ⁇ x ⁇ ⁇ y ⁇ ⁇ e wg 2 ⁇ ( x , y ) ⁇ ⁇ x ⁇ ⁇ y ,
  • FIG. 2 a diagram of the coupling efficiency ⁇ (in ordinate) as a function of the ratio S f /S wg (in abscissa, logarithmic scale) is shown.
  • the integrated waveguide 101 has a refractive index contrast ⁇ defined as:
  • ⁇ 2 ⁇ n core - n lc - n uc n lc + n wc .
  • the refractive index contrast ⁇ depends on the refractive indexes n core , n 1c and n uc ; in the exemplary case that the lower cladding and the upper cladding are made of SiO 2 , a SiON waveguide core of refractive index equal to 1.4645 corresponds to a refractive index contrast ⁇ of approximately 1%, while a SiON waveguide core of refractive index equal to 2 corresponds to a refractive index contrast ⁇ of approximately 40%,
  • a rib waveguide having a rib of width W 0 , an height h and a slab height t, supports a mode with a vertical dimension S v wg at 1/e, a horizontal dimension S h wg at 1/e, and an average mode size S wg equal to:
  • K S f /S wg is the ratio of the fields dimensions, and it is K ⁇ 1.
  • the waveguide structure of FIG. 1 it is possible to maximize the coupling efficiency and, at the same time, decrease the fiber-to-waveguide alignment sensitivity. This can be achieved by properly varying the values of the parameters L, W, L tip , h and t.
  • the coupling efficiency between the modes in the optical fiber and in the waveguide can be maximized by properly choosing the values for the width W of the waveguide tip and the height t of the slab 113 a.
  • FIG. 3 a diagram showing the variation of the field vertical dimension S v wg and the field horizontal dimension S h wg (both in ordinate) at the interface facet 119 with the waveguide tip width W is presented.
  • both the vertical dimension S v wg and the horizontal dimension S h wg of the field vary with the waveguide tip width W; in particular, by decreasing the width W, the field horizontal dimension S h wg increases accordingly, tending to infinity as the width W tends to zero; on the contrary, the field vertical dimension S v wg increases up to a value substantially equal to the vertical dimension of the field in the slab 113 a , and, if the waveguide is symmetrical, cannot be increased any further.
  • the field vertical dimension S v wg tends to infinity when the width W tends to zero.
  • the Applicant has carried out numerical investigations on the waveguide structure of FIG. 1 in order to establish the values of the waveguide parameters that maximize the coupling efficiency with the optical fiber mode, and the results of these investigations are reported hereinbelow.
  • FIGS. 4A , 4 B and 4 C diagrams showing contour plots of the coupling losses (defined as the one-complement 1 ⁇ of the coupling efficiency ⁇ ), in dB, as a function of the two waveguide geometrical parameter ratios W/W 0 (in abscissa) and t/h (in ordinate) are depicted.
  • the diagrams have been obtained by calculating the overlap integral between the optical field in the waveguide at the interface facet 119 , simulated by a simulator based on the beam propagation method, and the circular gaussian field of an optical fiber. It is pointed out that the diagrams of FIGS.
  • ratios W/W 0 and t/h cannot be chosen equal to the optimum, for example, for satisfying particular circuit requirements, the designer need to use a slab height t such that, in combination with a given waveguide height h, the ratio t/h is different from the optimum value, the coupling losses can still be kept below desired levels by choosing values of the parameters W, W 0 , t, and h such that the ratios W/W 0 and t/h are within prescribed ranges, which depends on the refractive index contrast. For example, considering again the diagram of FIG.
  • FIG. 5A reports contour plots of the coupling losses calculated by averaging the results reported in the diagrams of FIGS. 4A , 4 B and 4 C and, in FIG. 5B , a diagram showing the standard deviations of the coupling losses values in the different regions of the plane (W/W 0 ; t/h) is shown.
  • the structure has a low sensitivity to variations of the refractive index contrast ⁇ (at least, within the chosen range of variability) for a constant fields dimensions ratio K (equal to 1.44).
  • K constant fields dimensions ratio
  • the refractive index contrast ⁇ varied between 2% and 8%; however, the Applicant has observed that similar results are obtained even in the case the refractive index contrast ⁇ takes values significantly higher than 8%, for example 20% or even more (theorically, these results can be obtained for any refractive index contrast ⁇ , provided that the value of K is suitable, as discussed below);
  • the laterally-tapered transition waveguide core rib portion 117 b can be designed in a conventional way, so as to avoid abrupt transitions between the narrower tip waveguide core rib portion 117 c and the wider circuit waveguide core rib portion 117 a .
  • the length L of the transition waveguide core rib portion 117 b is chosen to be of the order of the hundreds of microns.
  • the length L of the transition waveguide core rib portion 117 b is chosen greater than a minimum value Lmin defined as:
  • L 0 is the minimum length of the transition waveguide core rib portion 117 b that guarantees an adiabatic transition even in case that the width W of the tip waveguide core rib portion 117 c is chosen to be equal to zero and the area of the interface facet 119 reduce to zero, thereby the interface of the waveguide to the external field reduces to the slab 113 a only.
  • the Applicant has observed that waveguide transition portions shorter than 500 ⁇ m are capable of ensuring a good adiabatic transformation of the optical field from the wider circuit waveguide portion to the narrower tip waveguide portion. Adiabatic transitions are not prevented by the use of longer waveguide transition portions, but no additional benefits have been observed that could justify a greater occupation of area. Thus, the Applicant has taken 500 ⁇ m as the lower limit L 0 of the length of the transition portion in the most critical case of a width W reduced to zero.
  • the length L tip of the tip waveguide core rib portion 117 c is chosen to be of the order of the hundreds of microns, and the effective length of this waveguide core is rib portion is determined by taking into account the technological tolerances in cutting the wafer into individual dies and in preparing the chip edge face.
  • L tip is chosen to be equal to or greater than 100 ⁇ m.
  • transition waveguide core rib portion 117 b is sufficiently long and W/W 0 is near 1, there is no reason for having a long tip waveguide core rib portion 117 c to protect the structure from technological tolerances; on the contrary, a suitable guard has to be provided when short transition regions and small W/W 0 values are considered. For these reasons, the following value for the length of the tip waveguide core rib portion 117 c is considered:
  • L tec depends on the technological tolerance in cutting the wafer into dies and in preparing the chip edge face.
  • the Applicant has found that a reasonable value for L tec is 300 ⁇ m and this is the maximum value that guarantees negligible propagation losses.
  • the integrated waveguide structure of FIG. 1 shows two other important properties.
  • the input optical fiber is coupled to the tip waveguide core rib portion 117 c , which ensures monomodality thanks to the extremely small cross section thereof.
  • This fact guarantees that only the fundamental mode is excited in the circuit waveguide circuit waveguide core rib portion 117 a , i.e., in the circuit waveguide, irrespective of any possible misalignment between the fiber and the waveguide.
  • This feature becomes extremely useful when the circuit waveguide is dimensioned to have a cross-sectional area close to, or even above the second guided mode cut-off (case in which a two mode propagation is possible), but only the fundamental mode excitation is desired.
  • the laterally-tapered transition waveguide core rib portion 117 b had a cubic profile, and the integrated waveguide has been coupled to a small-core optical fiber with average mode dimension at 1/e (S f ) equal to 3.6 ⁇ m.
  • the circuit waveguide average mode dimension at 1/e. (S wg ) was determined to be equal to 2.6 ⁇ m; consequently, the value of K was 1.38.
  • the optical fiber has also been coupled to a second integrated waveguide structure without the mode spot-size conversion structure of FIG. 1 , i.e., an integrated waveguide coinciding with the circuit waveguide of FIG. 1 .
  • the measured coupling loss amounted to 0.8 dB.
  • FIGS. 7A and 7B report the measured coupling efficiencies (in ordinate, dB scale) as a function of the fiber horizontal ( FIG. 7A ) and vertical ( FIG. 7B ) misalignments (in abscissa, ⁇ m) in the: two experimental cases discussed above.
  • the curve s have been normalized to their maximum value. It is clear that the waveguide structure of FIG. 1 is less sensitive to the alignment, especially along the vertical axis.
  • the integrated waveguide structure of FIG. 1 can be employed in the realization of any integrated optical component.
  • an integrated optical component 821 is schematically shown comprising, integrated in a chip 807 , a ring filter 823 , particularly, but not at all limitatively, a filter for high bit rate applications operating at a wavelength equal to 1550 nm.
  • the ring filter 823 comprises an integrated waveguide similar to the circuit waveguide portion shown in FIG. 1 .
  • a waveguide 801 is integrated in the chip 807 .
  • the waveguide 801 has the structure shown in FIG. 1 , and includes an input mode spot-size converter 825 a , an output mode spot-size converter 825 b and, interposed therebetween, a circuit waveguide section 827 arranged in respect to the ring filter 823 so as to form a directional coupler 829 .
  • the input and output mode spot-size converters 825 a and 825 b are respectively coupled to an input and an output optical fiber 805 a , 805 b.
  • W 0.8 ⁇ m
  • L tip 200 ⁇ m
  • t the slab height
  • the waveguide structure of FIG. 1 can be thus employed in any integrated optical component to enable high fiber coupling efficiencies and, at the same time, meet other requirements that must be satisfied.
  • the main advantages of the described waveguide structure are the capability of achieving a high coupling efficiency with an appropriate optical fiber, at the same time satisfying requirements on the waveguide characteristics different from the coupling efficiency, e.g. requirements imposed by the particular integrated optical device or devices to be formed and with which the waveguide has to interact (circuital requirements), weak influence on the coupling efficiency by tolerances on geometrical and optical parameters, low sensitivity to fiber-to-chip alignment, and selective fundamental mode excitation, even when multimode (in particular, two-mode) circuit waveguides are employed.
  • the described waveguide structure is particularly adapted for integrated waveguides characterized by medium to high refractive index contrast values, particularly refractive index contrast values from approximately 1% to approximately 20%. Extremely good results are achieved if the described waveguide structure is realized with materials ensuring an index contrast value from approximately 5% to approximately 7%. It is observed that these index contrast values are adapted to realize integrated optical devices for Wavelength Division Multiplexing (WDM) and Dense WDM (DWDM) communication systems. With such index contrast values, waveguides with very small bending radii can be formed, and compact devices such as ring filters (as the one shown in FIG. 8 ) and Mach-Zehnder interferometers with useful free spectral ranges can be obtained.
  • WDM Wavelength Division Multiplexing
  • DWDM Dense WDM
  • ring filters with a free spectral range of 100 GHz need bending radii lower than 300 ⁇ m, and can be realized only if the index contrast is at least equal to approximately 5%.
  • the described waveguide structure can be expediently exploited also for higher refractive index contrast values, up to approximately 40%.
  • the interval of refractive index contrast values for which the described waveguide structure may be exploited depends on the ratio K between the dimension of the optical field supported by the waveguide and the dimension of the external optical field to be coupled to the waveguide field: as long as this ratio is relatively low, and particularly within approximately 1 and 3, any refractive index contrast value is suitable.
  • the described waveguide structure is symmetrical, and can be exploited in correspondence of both optical inputs and optical outputs of integrated optical devices.
  • the invention can be applied in general whenever an integrated waveguide has to be coupled to an external optical field, either guided or not, and, particularly, an external optical field such that the ratio K of the dimensions thereof to the dimensions of the field supported by the integrated waveguide is relatively low, and preferably falls within the range from approximately 1 to approximately 3.
  • the waveguide structure according to the present invention is easy to fabricate. Thanks to the fact that only a lateral tapering of the waveguide rib core is present, the mode spot size conversion structure can be realized at the same time the rib core 113 b is defined, by means of the same photolithography; no additional manufacturing steps are required compared to the manufacturing on a rib waveguide, only a peculiar layout of the photolithographic mask. This is a great advantage with respect to two-dimensional tapering known in the art, which involve more complicated processes with more steps. Alternative fabrication methods are however possible.

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US11275210B1 (en) * 2018-12-07 2022-03-15 PsiQuantum Corp. Waveguide couplers for multi-mode waveguides
US11635570B1 (en) 2019-02-08 2023-04-25 PsiQuantum Corp. Multi-mode multi-pass delay
US11391890B1 (en) 2019-05-15 2022-07-19 PsiQuantum Corp. Multi-mode spiral delay device
US11789205B1 (en) 2019-05-15 2023-10-17 PsiQuantum Corp. Multi-mode spiral delay device
US12066661B2 (en) 2019-05-15 2024-08-20 Psiquantum, Corp. Multi-mode spiral delay device
US10921518B2 (en) * 2019-05-23 2021-02-16 International Business Machines Corporation Skewed adiabatic transition
US11539107B2 (en) 2020-12-28 2022-12-27 Waymo Llc Substrate integrated waveguide transition including a metallic layer portion having an open portion that is aligned offset from a centerline
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CN115267972A (zh) * 2022-08-18 2022-11-01 吉林大学 一种基于聚合物/二氧化硅复合芯层结构的模斑转换器

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